Fish + fish = 2 fish when you're talking about math, but your friend is right, too. "Fish" is a noun that's both singular AND plural. So, your friend is saying that (one) fish + (one) fish = (two/more than one/plural) fish. So, both of you are correct, in different ways (math and English). However, if the the "fish" and "fish" were used plurally, you would be incorrect because you thought of them as singular. But, because no one specified whether or not it's singular or plural, you are correct, from your perspective. Kapiche?